Calibration Aspects of Multiple Line-scan Vision System Application for Planar Objects Inspection

Abstract

The System Set-up Time it is one of the characteristics of an Industrial Vision System, besides the accuracy performances and response time. Minimizing the set-up time while keeping the performances in accuracy and in response time is one of the goals of any advanced Vision System. Starting from the purpose and the required performances of the proposed Industrial Vision System, in the paper is presented a calibration method developed for a multiple line-scan camera Vision System (in particular for a dual line-scan camera system). The calibration method presented is based on analyzing the image of a calibration tool exposed to the Vision System. There are presented the type of dimensional distortions identified from the experimental results. The second part of the paper presents the calibration method. The Industrial Vision System described in the paper is designed for silhouette inspection of planar objects located on a moving scene (transport conveyor), in a robotic handling application (it is a pure 2D Vision System, the volumetric characteristics of the analyzed objects being not relevant for the application). However the height of the object is varying in time (from one set of objects to another). Due to the fact the distance between the cameras and the objects is changing, the measuring results are affected. The proposed calibration method allows the Vision System to self adjust the calibration parameters for a known change in height of the objects, without affecting the accuracy system performances. In the final section of the paper are presented some practical aspects of the proposed calibration method, and the balance between the off-line and the online required computational efforts from the Vision System. 1 MULTI LINE-SCAN CAMERA VISION SYSTEMS CHARACTERISTICS The class of the Artificial Vision Systems dedicated for analyzing objects located on a moving scenes (conveyor) presents some specific characteristics relative to the Artificial Vision Systems dedicated for static scenes. These characteristics are identified also on the Image Calibration Process (Borangiu, et al., 1994)., (Haralick and Shapiro, 1992). Figure 1 presents the model of the image obtained from a dual line-scan camera Vision System. For this class of the Artificial Vision Systems we could identify as relevant for the calibration process the following characteristics: The system is using line-scan cameras for the image acquisition. The system is a dual-camera. The obtained image has significant distortions on (and only on) the image sensors direction. There is an overlapped image area between the two cameras. The end of the acquisition line of the 1 camera is overlapping the beginning of the acquisition line of the 2 camera. This overlapping area is significant in dimension and is a constant parameter resulted during the artificial vision system installation process. There is a lengthwise conveyor distance between the acquisition lines of the two cameras. This distance is also a constant parameter and its value is fixed during the system installation process. 36 Hossu A. and Hossu D. (2008). CALIBRATION ASPECTS OF MULTIPLE LINE-SCAN VISION SYSTEM APPLICATION FOR PLANAR OBJECTS INSPECTION. In Proceedings of the Fifth International Conference on Informatics in Control, Automation and Robotics RA, pages 36-40 DOI: 10.5220/0001480300360040 Copyright c © SciTePress 2 THE PATTERN BASED CALIBRATION TOOL For the calibration process we adopted the method of using a Pattern based Calibration Tool. This Pattern based Calibration Tool represent a set of blobs with a priori known dimensions and locations for the real world (millimeters and not image pixels) (Croicu, et al., 1998). The outcome of using this type of calibration technique was to obtain the following: Estimation with the highest accuracy of the scene model parameters on the distortions direction Estimation of the size of the overlapped image area for both cameras The parallelism of the two acquisition lines if obtain during the installation process, using the support of the Calibration Tool The accuracy of installing the cameras in such a way to obtain the perpendicularity of the acquisition lines on the moving direction of the scene (of the conveyor). Obtain a high accuracy on the distance lengthwise the conveyor of the acquisition lines. The shape and the dimensions of the pattern adopted for the Calibration Tool force this characteristic. 3 CALIBRATION TOOL DESCRIPTION In Figure 2 it is presented the pattern adopted for the Calibration Tool used for the dual line-scan camera Vision System (the dimensions are presented in millimeters) (Croicu, et al., 1998)., (Hossu, et al., 1998). The characteristics of the adopted Pattern: The pattern contains dark blebs (marks) placed on a bright background (with a high level of light intensity for the image) The pattern is symmetrical on the vertical direction (lengthwise the conveyor). The two cameras have the acquisition lines parallel one each other but located on different position on the conveyor (due to the lighting system adopted – built from two fluorescent tubes used for obtaining the image from the reflection on the object surface). 1st Camera will have the acquisition line located on the top edge of the lower section of the pattern, and the 2nd Camera will locate its acquisition line on the bottom edge of the upper section of the pattern. The pattern is partially homogenous on the horizontal axis (the direction crosswise the conveyor, the direction of the distortions) The pattern contains a characteristic of a small difference (1 mm.) between the even and the odd marks. This will force the installation process to be very accurate in obtaining the parallelism of the acquisition lines of the cameras and also the perpendicularity on the conveyor direction. Figure 1: The image obtained from a dual line-scan camera Vision System. CALIBRATION ASPECTS OF MULTIPLE LINE-SCAN VISION SYSTEM APPLICATION FOR PLANAR OBJECTS INSPECTION